You have no End If for any If statement and you should use ElseIf for addidtinnal checks. What wrapper are you using, you may want to use a referrer also?
Code:
Private Sub Command1_Click()
Dim strHTML As String
strHTML = Wrapper.PostWrapper("http://www.neopets.com/login.phtml", "username=" & txtuser.Text & "&password=" & txtpass.Text & "&destination=%2petcentral.phtml/")
If InStr(1, strHTML, "badpassword") Then
Label1.Caption = " Invalid Password/Username "
ElseIf InStr(1, strHTML, "this account has been ") Then
Label1.Caption = " this account has been frozen "
ElseIf InStr(1, strHTML, "petcentral") Then
Label1.Caption = " Successful Login "
Else
Label1.Caption = " unknown error "
Exit Sub
End If
End Sub